Any PCIe card will work in ANY PCIe slot as long as it is long enough to support the card. For example, I can take a PCIe X1 card and put it into a a PCIe x16 slot without issues. The card will work and will only run at X1 speed (for obvious reasons). Also, it doesn't matter what slot you put it in either.
Most modern RAID cards will support 3TB without issues. Do you have a budget? RAID cards and HBAs can get REALLY expensive REALLY fast. I have easily seen some go for +1k USD no problem.
If motherboard is good enough then ANY RAID controller or HBA will work properly in the first x16 slot. For a while I was running my X99 Classy with VGA in last slot RAID card in first - no problems.
Technically any RAID controller of any major vendor Adaptec/PMC, LSI or Areca should support 4TB+ even on really old models. I have still in use simple Adaptec 2805 with multiple 5TB drives - no issues to report.
Just, if needed, upgrade firmware to required level.
Like Lord Xeb said. First answer yourself if you really need RAID. If yes then pricing climb really, really fast. If you just need superior redundancy then go with HBA for drive pool setup - no RAID level can beat that solution and pricing for 16-20 port models is fairly reasonable. On personal note, running RAID only on SSDs now and HDDs in pools. Rebuild time on xyz TBs of hard drives take days/weeks if array is really big. It's just madness. IMHO RAID usefulness is already at breaking point with spinning media.
You can even put in a x16 card into a x8, an x4, or an x1 slot and it will still work, as long as the slot has a notch in the back instead of plastic blocking the card from being inserted. You can even tape off the appropriate pins on >x1 cards and put them into their designated slot and it will still work.
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